Heat exchanger

ABSTRACT

A heat exchanger is provided having two headers with a plurality of elongate tubes extending between the headers, where the tubes cooperate with an array of fins. The tubes have a plurality of dimpled sections having a non-uniform dimple density alternating with a plurality of smooth sections. The alternating dimpled and smooth sections allow heat to effectively transfer from a liquid coolant flowing through the tubes into the tube walls and fins at laminar, transitional, and turbulent flow conditions.

TECHNICAL FIELD

This invention relates to a heat exchanger of a tube and fin design.

BACKGROUND

Tube and fin heat exchangers are used to transfer heat between a liquid coolant flowing through the tubes and the surrounding environment. The tube and fin design consists of several tubes that extend between first and second headers with the tubes cooperating with an array of fins providing a large surface area. The liquid coolant flows across the tubes from one header to the other while the fins are exposed to the air of the surrounding environment. The fins and tubes are made from a material having a high thermal conductivity allowing heat to effectively transfer between the liquid coolant and the air of the surrounding environment. The headers are typically attached to tanks that collect the liquid flowing in and out of the heat exchanger.

In order to transfer heat from the tubes and fins to the air, energy must first be transferred from the liquid coolant to the tubes and fins. Various tube and fin designs have been proposed to increase heat transfer from the liquid to the tubes and fins, however, various designs function differently at different flow rates. Both smooth tubes and tubes having dimples that protrude into the interior of the tube have been used in tube and fin heat exchangers. During times of low flow rates when the flow is laminar, the difference in the amount of the heat transfer of a tube and fin heat exchanger having either a smooth tube design or a dimpled tube design is negligible. When the liquid coolant flow rate increases, flow enters a transitional flow phase, between laminar and turbulent flow, where the heat transfer from the liquid coolant to the tube walls and fins is significantly greater with dimpled tubes. Higher heat transfer is achieved with a heat exchanger using dimpled tubes rather than smooth tubes during transitional flow, because the dimples “stir up” the flows creating disturbances, which increases the turbulence and heat transfer. During turbulent flow, the heat transfer from the liquid coolant to the tube walls and fins is significantly greater with smooth tubes rather than dimpled tubes. Higher heat transfer is achieved with a heat exchanger using smooth tubes rather than dimpled tubes during turbulent flow, because the flows already have significant disturbances, and the dimples on the walls of the dimpled tubes reduce the contact area between the tubes and fins reducing the heat that transfers between the tubes and fins. The reduction in heat transfer from the tubes to the fins ultimately reduces the overall heat transferred from the liquid coolant in the heat exchanger to the surrounding external environment.

It would be desirable to provide a heat exchanger that encompasses the properties of the dimpled tubes during transitional flow and smooth tubes during turbulent flow, to allow for increased heat transfer during both flow conditions of the liquid coolant.

SUMMARY

A heat exchanger is disclosed for transferring heat between a liquid coolant and the air of the surrounding environment. The heat exchanger includes several elongate tubes that extend between two headers and cooperate with an array of fins. The tubes consist of alternating dimpled and smooth sections where the dimpled sections have a non-uniform dimple density. Alternating the dimpled and smooth sections on the tubes, allows for sufficient heat transfer from the liquid flowing through the tube and into the tube walls and fins at laminar, transitional, and turbulent flow conditions. The liquid coolant flows across the tubes from one header to the other while the fins are exposed to the air of the surrounding environment. The tubes and fins are made from a material having a high thermal conductivity allowing heat to effectively transfer from the liquid coolant, into the tubes and fins, and into the surrounding environment. Preferably, tanks are attached to each header to collect the liquid coolant that flows in and out of the tubes. The heat exchanger may also work in the reverse direction, where the heat is flowing from the surrounding environment, into the tubes and fins, and into the liquid coolant that is flowing through the tubes, as in the case of an evaporator in an air conditioning system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of the heat exchanger, the number of tubes shown is reduced, and the spacing between sections of each of the accordion style fins is increased for ease of illustration;

FIG. 2 is a partial cross-sectional isometric view of the header, tubes, and fins of the heat exchanger taken along the line 2-2 in FIG. 1;

FIG. 3 is a longitudinal cross-sectional view of the tube taken along the line 3-3 of FIG. 2;

FIG. 4 a is a plan view of a tube having zones of dimpled sections with a gradually decreasing dimple density, and alternating smooth zones;

FIG. 4 b illustrates a graph having a plot of the dimpled density of the tube in FIG. 4 a versus the tube length in the direction X;

FIG. 5 a is a plan view of a tube having zones of clustered dimpled sections, and alternating smooth sections;

FIG. 5 b illustrates a graph having a plot of the dimpled density of the tube in FIG. 5 a versus the tube length in the direction X; and

FIG. 6 illustrates a graph comparing the relative heat transfer capabilities of dimpled tubes, smooth tubes, and tubes with alternating dimpled and smooth sections during laminar, transitional, and turbulent flow conditions.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.

FIG. 1 illustrates a heat exchanger 10 according to the present disclosure. The heat exchanger 10 consists of several elongate tubes 12 that extend between two headers 14, with the tubes 12 attached to an array of fins 16. In the preferred embodiment, a liquid coolant flows across the tubes 12 from one header 14 to the other in a single pass. The invention however, is not limited to a single pass flow and the disclosure should be construed to include other embodiments having multiple passes. Also, in the preferred embodiment, the fins 16 are disposed between adjacent tubes, arranged in an accordion type configuration, and exposed to the air of the surrounding environment. The invention however, is not limited to the according type fin configuration, and should be construed to include other fin configurations, such as an intersecting tube and fin configuration. The tubes 12, headers 14, and fins 16 are preferably made from a material having a high thermal conductivity, such as aluminum, copper, brass, or steel, but should not be limited to these materials. Brazing is typically the process used to fix the tubes 12 to the headers 14. Two header tanks 18 are provided that are fixed to the headers 14 and collect the liquid coolant flowing in and out of the heat exchanger 10 at ports 20. Preferably, the header tanks 18 are made from a lightweight corrosion resistant material such as plastic. However, the header tanks 18 should not be limited to lightweight plastic only, and could be made from other materials such as aluminum, steel, or copper alloys such as brass.

As illustrated in FIG. 2, the elongate tubes 12 have an outside width W and an outside height H. Preferably, the width W of the elongate tubes 12 ranges from 10 mm to 40 mm and the height H ranges from 1 mm to 2.5 mm. Preferably the material thickness of the elongate tubes 12 will range between 0.15 mm and 0.35 mm. The headers will have a preferred material thickness of about 1.5 mm, while the fins 16 will have a preferred material thickness that ranges from 0.05 mm to 0.10 mm. The fin height will preferably range from 4 mm and 9 mm. The fin height being approximately equal to the distance between two adjacent elongate tubes 12 in the accordion fin design illustrated.

Referring to FIGS. 2 and 3, dimples 22 are provided that protrude into the interior of the elongated tubes 12. The dimples 22 increase heat transfer during transitional flow conditions by agitating the liquid coolant. The agitation is represented by the circular shaped flow arrows in FIG. 3.

Referring to FIGS. 4 a and 5 a, an elongate tube 12 is divided into alternating dimpled sections 24 and smooth sections 26. The smooth tubes 26 have higher heat transfer than the dimpled sections 24 during turbulent flow conditions.

Still referring to FIGS. 4 a and 5 a, the preferred embodiment alternates dimpled sections 24 with smooth sections 26 in order to take advantage of the increased heat transfer properties of both dimpled and smooth tubes whether in transitional or turbulent flow conditions. The dimpled sections have a length L and the smooth sections have a length M. Preferably, the lengths L and M will both range between 10 mm and 200 mm, and more preferably from 35 mm to 75 mm.

FIGS. 4 a and 4 b illustrate an embodiment according to the present disclosure having alternating dimpled sections 24 and smooth sections 26 of an elongate tube 12, where the density of dimples 22 in the dimpled sections 24 gradually decreases as you move in a direction X. FIG. 4 b is a graphical representation of a portion of the elongate tube 12, showing the dimple density decreasing in a linear fashion over each dimpled section 24 and a dimple density of zero in each smooth section as you move in a direction X.

FIGS. 5 a and 5 b illustrate an alternative embodiment according to the present disclosure having alternating dimpled sections 24 and smooth sections 26 of an elongate tube 12. The dimples 22 are arranged in several clusters, where the number of dimples in each cluster decreases as you move in a direction X across each dimpled section 24. FIG. 5 b is a graphical representation of a portion of the elongate tube 12, showing clusters of decreasing size over each dimpled section 24, where the dimple density is zero between each cluster and in each smooth section as you move in a direction X.

The present disclosure should not be limited to the embodiments described herein, and should be construed to include all elongate tubes 12 having alternating dimpled sections 24 and smooth sections 26, where the dimple density in the dimpled sections 24 is non-uniform.

The graph in FIG. 6 illustrates the relative heat transfer capabilities of dimpled, smooth, and tubes having alternating dimpled and smooth sections during laminar, transitional, and turbulent flow conditions.

While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. As previously noted, the invention is not limited to radiators, but can be a heat exchanger used as a condenser or evaporator in an air conditioning system or the like. The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention. 

What is claimed is:
 1. A heat exchanger comprising: first and second headers having a plurality of elongate tubes extending there between, the tubes cooperating with an array of fins, wherein the tubes have a plurality of dimpled sections having a non-uniform dimple density followed by one of a plurality of alternating smooth sections, in order to effectively transfer heat between a liquid flowing through each tube and a tube wall at laminar, transitional, and turbulent flow conditions.
 2. The heat exchanger of claim 1, wherein the dimpled sections have a first end and a second end, and the density of the dimples gradually decreases along a longitudinal direction moving from the first end to the second end in the direction of liquid flow.
 3. The heat exchanging tube of claim 1, wherein each of the tubes have at least three dimpled sections and at least three smooth sections.
 4. The heat exchanger of claim 1, wherein the dimples within the dimpled sections are arranged in a plurality of clusters.
 5. The heat exchanger of claim 1, wherein the plurality of elongate tubes are made from aluminum.
 6. The heat exchanger of claim 1, further comprising two header tanks that are fixed to the first and second headers, wherein the header tanks collect the liquid that flows through the plurality of elongate tubes.
 7. The heat exchanger of claim 6, wherein the header tanks are made from plastic.
 8. The heat exchanger of claim 6, wherein the header tanks are made from aluminum.
 9. The heat exchanger of claim 1, wherein the liquid flows into the heat exchanger at the first header, through the plurality of tubes, and out of the heat exchanger at the second header.
 10. The heat exchanger of claim 1, wherein the dimpled sections have a length L that ranges between 10 mm and 200 mm.
 11. The heat exchanger of claim 1, wherein the smooth sections have a length M that ranges between 10 mm and 200 mm.
 12. A heat exchanger comprising: first and second headers having a plurality of elongate tubes extending there between, the tubes cooperating with an array of fins; and first and second header tanks having ports, attached to the first and second headers for collecting a liquid flowing into and out of the heat exchanger, wherein the tubes have a plurality of dimpled sections having a non-uniform dimple density followed by one of a plurality of alternating smooth sections, in order to effectively transfer heat between the liquid flowing through each tube and a tube wall at laminar, transitional, and turbulent flow conditions.
 13. The heat exchanger of claim 12, wherein the dimpled sections have a first end and a second end, and the density of the dimples gradually decreases along a longitudinal direction moving from the first end to the second end in the direction of liquid flow.
 14. The heat exchanger of claim 12, wherein each of the tubes have at least three dimpled sections and at least three smooth sections.
 15. The heat exchanger of claim 12, wherein the dimples within the dimpled sections are arranged in a plurality of clusters.
 16. The heat exchanger of claim 12, wherein the plurality of elongate tubes are made from aluminum.
 17. The heat exchanger of claim 12, wherein the header tanks are made from plastic.
 18. The heat exchanger of claim 12, wherein the header tanks are made from aluminum.
 19. The heat exchanger of claim 12, wherein the dimpled sections have a length L that ranges between 10 mm and 200 mm.
 20. The heat exchanger of claim 12, wherein the smooth sections have a length M that ranges between 10 mm and 200 mm. 